analysis: object modeling - technische universität münchen · do uml associations have direction?...

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Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 2

Exercise 2.6

Draw a sequence diagram for the warehouseOnFire scenario (asdescribed in the requirements elicitation lecture).

Include the objects bob, alice, john, system, and instances of otherclasses you may need.

Draw only the first five message sends.


Solutions to exercise 2.6

:FRIEND

:EmergencyFormreportEmergency()

new EmergencyForm()

specifyIncident()

notifyDispatcher()

alice john

requestResource(fireTruck)

commit()

bob

Initiating actor shouldbe on this side.

Dispatcher should beon this sideAll objects involved in

this scenario are instances.This is how an objectcreation looks like.

Note: this exercise can have many other acceptable solutions.


Exercise 2.7

Draw a sequence diagram for the ReportIncident use case (asdescribed in the requirements elicitation lecture).

Make sure it is consistent with the sequence diagram of the previousexercise.

Draw only the first five message sends.


Sample solution to exercise 2.7

FRIEND

EmergencyFormreportEmergency()

new EmergencyForm()

specifyIncident()

notifyDispatcher()

FieldOfficer Dispatcher

*requestResource()

commit()

This is a use case,hence, we are dealing

with classes.

Class and operationnames should be the

same as in the previousdiagram.

This is how an iterationis specified.

Note: this exercise can have many other acceptable solutions, as before.


Outline

♦ From use cases to objects

♦ Object modeling

♦ Class vs instance diagrams

♦ Attributes

♦ Operations and methods

♦ Links and associations

♦ Examples of associations

♦ Two special associationsw Aggregationw Inheritance


From Use Cases to Objects

Level 1 Use Case

Level 2 Use Cases

Level 3 Use Cases

Operations

ParticipatingObjects

Le vel 2

Level 1

Le vel 2

Le vel 3 Le vel 3

Le vel 4 Le vel 4

Le vel 3

A B

UserTasks


From Use Cases to Objects: Why FunctionalDecomposition is not Enough

Scenarios

Level 1 Use Cases

Level 2 Use Cases

Operations

ParticipatingObjects

Le vel 2

Level 1

Le vel 2

Le vel 3 Le vel 3

Le vel 4 Le vel 4

Le vel 3

A B


How do we describe complex systems (Natural Systems, SocialSystems, Artificial Systems)?

Epistemology

Knowledge about Causality(Dynamic Model)

Describes our knowledge about the system

Knowledge about Functionality(Functional model)

Knowledge about Relationships(Object model)

NeuralNetworks

DataFlow Diagrams(SA/SD)

Scenarios/Use Cases(Jacobson)

FormalSpecifications

(Liskov)

State Diagrams(Harel)

Petri Nets(Petri)Inheritance

Frames,SemanticNetworks (Minsky)

Uncertain KnowledgeFuzzy Sets (Zadeh)

Data Relationship(E/R Modeling, Chen)

Hierarchical Database

Model (IMS)

NetworkDatabase

Model(CODASYL)

RelationalDatabase Model

(Codd)

Fuzzy Frames(Graham)

Class Diagrams(“E/R + Inheritance”,

Rumbaugh)

SequenceDiagrams

Activity Diagrams


Definition: Object Modeling

♦ Main goal: Find the important abstractions

♦ What happens if we find the wrong abstractions?w Iterate and correct the model

♦ Steps during object modelingw 1. Class identification

t Based on the fundamental assumption that we can find abstractions

w 2. Find the attributesw 3. Find the methods

w 4. Find the associations between classes

♦ Order of stepsw Goal: get the desired abstractions

w Order of steps secondary, only a heuristicw Iteration is important


Class Identification

♦ Identify the boundaries of the system

♦ Identify the important entities in the system

♦ Class identification is crucial to object-oriented modeling

♦ Basic assumption:w 1. We can find the classes for a new software system (Forward

Engineering)

w 2. We can identify the classes in an existing system (ReverseEngineering)

♦ Why can we do this?w Philosophy, science, experimental evidence


Class identification is an ancient problem

♦ Objects are not just found by taking a picture of a scene or domain

♦ The application domain has to be analyzed.

♦ Depending on the purpose of the system different objects might befoundw How can we identify the purpose of a system?

w Scenarios and use cases

♦ Another important problem: Define system boundary.w What object is inside, what object is outside?


What is This?


Pieces of an Object Model

♦ Classes

♦ Associations (Relations)w Part of- Hierarchy (Aggregation)w Kind of-Hierarchy (Generalization)

♦ Attributesw Detection of attributesw Application specific

w Attributes in one system can be classes in another systemw Turning attributes to classes

♦ Methodsw Detection of methodsw Generic methods: General world knowledge, design patterns

w Domain Methods: Dynamic model, Functional model


Object vs Class

♦ Object (instance): Exactly one thingw The lecture on November 2 on Software Engineering from 14:30 -16:00

♦ A class describes a group of objects with similar propertiesw IETM, Author, Corrosion, Work order

♦ Object diagram: A graphic notation for modeling objects, classes andtheir relationships ("associations"):w Class diagram: Template for describing many instances of data. Useful for

taxonomies, patters, schemata...w Instance diagram: A particular set of objects relating to each other. Useful

for discussing scenarios, test cases and examples

♦ Together-J: CASE Tool for building object diagrams, in particularclass diagramsw Tutorial on November 10


UML: Class and Instance Diagrams

Inspector

joe:Inspector

mary:Inspector

anonymous:Inspector

Class Diagram

Instance Diagram


Attributes and Values

name:stringage: integer

Inspector

name = “Joe”age = 24

joe:Inspector

name = “Mary”age = 18

mary: Inspector


Operation, Signature or Method? What when?

♦ Operation: A function ortransformation applied to objects in aclass. All objects in a class share thesame operations (Analysis Phase)

♦ Signature: Number & types ofarguments, type of result value. Allmethods of a class have the samesignature (Object Design Phase)

♦ Method: Implementation of anoperation for a class (ImplementationPhase)

Polymorphic operation: The sameoperation applies to many differentclasses.

Workorder

File_name: StringSize_in_bytes: integerLast_update: dateStickies: array[max]

print()delete()open()close()write()read()


Links and Associations

♦ Links and associations establish relationships among objects andclasses.

♦ Link:w A connection between two object instances. A link is like a tuple.

w A link is an instance of an association

♦ Association:w Basically a bidirectional mapping.

w One-to-one, many-to-one, one-to-many,w An association describes a set of links like a class describes a set of objects.


1-to-1 and 1-to-many Associations

Has-capital

One-to-one association

One-to-many association

City

name:String

Workorder

schedule()

StickyNote

x: Integery: Integerz: Integer

*

Country

name:String


Object Instance Diagram

Example for 1-to-many

:Sticky

x,y,z=(-1,0,5)

:WorkOrder :Sticky

x,y,z=(1,10,1)

:Sticky

x,y,z=(10,1,2)


Many-to-Many Associations

Work on **Mechanics Plane


Roles in Associations

♦ Client Role:w An object that can operate upon other objects but that is never operated

upon by other objects.

♦ Server Role:w An object that never operates upon other objects. It is only operated upon

by other objects.

♦ Agent Role:w An object that can both operate upon other objects and be operated upon

by other objects. An agent is usually created to do some work on behalf ofan actor or another agent.


Do UML associations have direction?w A association between two classes is by default a bi-directional mapping.

t Class A can access class B and class B can access class A

t Both classes play the agent role.

A B

If you want to to make A a client, and B a server, you can make theassociation unidirectional. The arrowhead points to the serverrole:

Class A ( the “client”) accesses class B (“the server”). B is also callednavigable

A Baccesses

Name DirectionName of association

Association Direction


Aggregation

♦ Models "part of" hierarchy

♦ Useful for modeling the breakdown of a product into its componentparts (sometimes called bills of materials (BOM) by manufacturers)

♦ UML notation: Like an association but with a small diamondindicating the assembly end of the relationship.


weight

Automobile

serial numberyearmanufacturermodelcolor

drivepurchase

Aggregation

Engine

horsepowervolume

onoff

3,4,5

Wheel

diameternumber of bolts

2,4

Door

openclose

Battery

ampsvolts

chargedischarge

*

Brakelight

onoff


Inheritance

♦ Models "kind of" hierarchy

♦ Powerful notation for sharing similarities among classes whilepreserving their differences

♦ UML Notation: An arrow with a triangleCell

MuscleCellBloodCell NerveCell

StriateSmoothRed White PyramidalCortical


Aggregation vs Inheritance

♦ Both associations describe trees (hierarchies)wAggregation tree describes a-part-of relationships (also called

and-relationship)w Inheritance tree describes "kind-of" relationships (also called

or-relationship)

♦ Aggregation relates instances (involves two or moredifferent objects)

♦ Inheritance relates classes (a way to structure thedescription of a single object)


Other Associations

♦ Uses:w A subsystem uses another subsystem (System Design)

♦ Contains:w Sometimes called “spatial aggregation”

w ... contains ...w Example: A UML package contains another UML package

♦ Parent/child relationship:w ... is father of ...w ... is mother of ...

♦ Seniority:w ... is older than ...w ... is more experienced than ...


Odds and Ends

♦ Hoererschein for the book:w Participating Bookstores: If you say you are taking this class, you get 10%

off at:t Buchladen am Obelisk (Barerstrasse),

t Kanzler (Gabelsbergerstrasse),

t Lachner (Theresienstrasse)

♦ Solution to last exercise

♦ Finding a superclass


Object Types11/2/00

♦ Entity Objectsw Represent the persistent information tracked by the system (Application

domain objects, “Business objects”)

♦ Boundary Objectsw Represent the interaction between the user and the system

♦ Control Objects:w Represent the control tasks performed by the system

♦ Having three types of objects leads to models that are more resilientto change.w The boundary of a system changes more likely than the control

w The control of the system change more likely than the application domain

♦ Object types originated in Smalltalk:w Model, View, Controller (MVC) => Observer Pattern


Example: 2BWatch Objects

♦ UML provides several mechanisms to extend the language

♦ UML provides the stereotype mechanism to present new modelingelements

<<entity>>Year

<<entity>>Month

<<entity>>Day

<<control>>ChangeDateControl

<<boundary>>LCDDisplayBoundary

<<boundary>>ButtonBoundary


Roles

♦ A role name is the name that uniquely identifies one end of anassociation.

♦ A role name is written next to the association line near the class thatplays the role.

♦ When do you use role names?w Necessary for associations between two objects of the same classw Also useful to distinguish between two associations between the same pair

of classes

♦ When do you not use role names?w If there is only a single association between a pair of distinct classes, the

names of the classes serve as good role names


Example of Role

Problem Statement : A person assumes the role of repairer with respect to another person, who assumes the role of inspector with respect to the first person.

Person* Creates Workorders

inspector

repairperson* Creates Workorders

Person Person

Person


Qualification

♦ The qualifier improves the information about the multiplicity of theassociation between the classes.

♦ It is used for reducing 1-to-many multiplicity to 1-1 multiplicity

With qualification: A directory has many files, each with a unique name

Without qualification: A directory has many files. A file belongs only to onedirectory.

Directory Filefilename

DirectoryFile

filename

1 *

0..11


Example

Problem Statement : A stock exchange lists many companies. However , a stock exchange lists only one company with a given ticker symbol. A company may be listed on many stock exchanges, possibly with different ticker symbols. Find company with ticker symbol AAPL, DCX.

StockExchangeCompany

tickerSym

* *lists


StockExchangeCompany

tickerSym

* *

Use of Qualification reduces multiplicity

StockExchange CompanytickerSym 0..11


How do you find classes?

♦ Learn about problem domain: Observe your client

♦ Apply general world knowledge and intuition

♦ Take the flow of events and find participating objects in use cases

♦ Apply design patterns

♦ Try to establish a taxonomy

♦ Do a textual analysis of scenario or flow of events (Abbott TextualAnalysis, 1983)

♦ Nouns are good candidates for classes


Mapping parts of speech to object model components[Abbot 1983]

Part of speech Model component Example

Proper noun object Jim Smith

Improper noun class Toy, doll

Doing verb method Buy, recommend

being verb inheritance is-a (kind-of)

having verb aggregation has an

modal verb constraint must be

adjective attribute 3 years old

transitive verb method enter

intransitive verb method (event) depends on


Example: Scenario from Problem Statement

♦ Jim Smith enters a store with the intention of buying a toy for his 3year old child.

♦ Help must be available within less than one minute.

♦ The store owner gives advice to the customer. The advice depends onthe age range of the child and the attributes of the toy.

♦ Jim selects a dangerous toy which is unsuitable for the child.

♦ The store owner recommends a more yellow doll.


Object Modeling in Practice: Class Identification

Foo

Balance

CustomerId

Deposit()Withdraw()GetBalance()

Class Identification: Name of Class, Attributes and Methods


Object Modeling in Practice:Encourage Brainstorming

Foo

Balance

CustomerId


Account

Balance

CustomerId


Naming is important!

“Dada”

Balance

CustomerId



Object Modeling in Practice

Account

Balance


Customer

Name

CustomerId

Find New Objects

CustomerIdAccountId

Iterate on Names, Attributes and Methods

Bank

Name


Object Modeling in Practice: A Banking System

Account

Balance


Customer

Name

CustomerId

CustomerIdAccountIdAccountIdBank

Name

Find New Objects

Iterate on Names, Attributes and Methods

Find Associations between Objects

Has

Label the assocations

Determine the multiplicity of the assocations

*


Object Modeling in Practice: Categorize!

SavingsAccount

Withdraw()

CheckingAccount

Withdraw()

MortgageAccount

Withdraw()

Customer

Name

CustomerId

Account

Amount


CustomerIdAccountIdAccountId

Bank

Name Has**


Avoid Ravioli Models

Customer

Name

CustomerId

Account

Amount



Bank

Name Has**

Don’t put too many classes into the same package:7+-2 (or even 5+-2)

Don’t put too many classes into the same package:7+-2 (or even 5+-2)


Avoid Ravioli Models: Put Taxonomies in a separate View

SavingsAccount

Withdraw()

CheckingAccount

Withdraw()

MortgageAccount

Withdraw()

Account

Amount




Object Modeling in Practice: Heuristics

♦ Explicitly schedule a team meeting for object identification

♦ Try to differentiate between entity, boundary and control objects

♦ Find associations and their multiplicityw Unusual multiplicities usually lead to new objects or categories

♦ Identify Aggregation

♦ Identify Inheritance: Look for a Taxonomy, Categorize

♦ Allow time for brainstorming , Iterate, iterate


Software Engineers are not the only System Analysts

OntologyObject and System boundary identification

PhenomenologyObjects are user defined

Idealism

Naive IdealismObjects exist only in my

imagination. If I close my eyes, they don't exist (Berkeley)

Critical IdealismReality is determined by

our ideas(Kant, Hegel, Schopenhauer)

Materialism

MarxIdeas are determined

by the economic reality

DialectismIdeas are determined by the dialog between the user and

reality(Sokrates, Hegel, Marx)

Kant Ideas are made up by humans"Ding an sich" :Reason for perception but can never be

seen itself

Schopenhauer

Realism

Naive RealismThings are exactly how we experience

them

PlatoReality can never be seen only its shadow.

Religion

David Hume

Goethe

Dualistic Idealism Monistic Idealism

Steiner


What is a Software Engineer?

♦ From the point of view of phenomenology, SoftwareEngineers are dialectic monistic idealists:w Idealists:

t They accept that ideas (called requirements or “customer’s wishlist”)are different from reality.

t The reality might not yet exist (“Vaporware is always possible ”)

wThey are monistic:t They are optimistic that their ideas can describe reality.

wDialectic:t They do this in a dialogue with the customer


Summary

In this lecture, we reviewed the construction of the object model fromuse case model. In particular, we described:

♦ Identification of objects

♦ Refinement of objects with attributes and operations

♦ Generalization of concrete classes

♦ Identification of associations

♦ Reduction of multiplicity using qualification.

In the next lecture, we describe the construction of the dynamic modelfrom the use case and object models.


5.6 Consider the object model below (adapted from [Jackson, 1995]):

Given your knowledge of the Gregorian calendar, list all the problemswith this model. Modify it to correct each of them.

Exercises

Year

Month

Week

Day


Exercises (cont’d)

5.7 Consider the object model of the previous exercise. Usingassociation multiplicity only, can you modify the model such that adeveloper unfamiliar with the Gregorian calendar could deduce thenumber of days in each month? Identify additional classes ifnecessary.


Next Steps

♦ For all students:w Tomorrow: Requirements Elicitation Tutorial (REQ/QOC)w Next Thursday: Configuration Management

w Next Friday: TogetherJ Tutorial

♦ For STARS students:w GUI Mockup due tomorrow on Notes Bboard

t 2-3 slides + name of presenter for each team

w GUI Mockup review on Monday 14:15

analysis: object modeling - technische universität münchen · do uml associations have direction?...

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